In the field of high-voltage overhead electrical installations, it is well-known to use current-measuring apparatus that uses the Faraday effect, in particular by means of optical fibers through which light rays pass.
It is recalled that the Faraday effect is a magneto-optical effect resulting from an electromagnetic wave interacting with matter in the presence of a field.
Under the action of a magnetic field in the same direction as the electromagnetic wave, some types of matter present optical activity expressed by induced non-reciprocal circular birefringence, i.e. by wave propagation speeds through the matter that are different depending on whether the wave has circular polarization that is dextrorotatory or levorotatory.
In order to measure an electrical current, the Faraday effect is used either on a linearly-polarized light wave which may be considered to be the sum of two circularly-polarized waves, one of which is dextrorotatory and the other of which is levorotatory, or else via two counter-propagating light waves that are both circularly polarized the same way (either levorotatory or dextrorotatory). In the first case, the rotation of the polarization plane is measured by polarimetry techniques, and, in the second case, the phase difference is measured by the "Sagnac" interferometry technique.
In both cases, the phase shift angle due to the Faraday effect is proportional to the instantaneous magnitude of the current to be measured.
The light waves used for performing the measurement are conveyed via an optical fiber which goes one or more times round the electrical conductor whose current is to be measured.
Measurement by using the Faraday effect would seem to be particularly advantageous for measuring currents in a grounded metal-clad type installation, or a shielded-type installation. Measuring in this way makes it possible to improve compactness considerably, since conventionally-used current transformers take up several tens of centimeters of axial space. Installing an optical fiber takes up only a few centimeters of axial space. Moreover, optical measurement is not sensitive to the electromagnetic influences that exist in a shielded installation. The following points are other arguments for installing optical-type current transformers in shielded installations:
they offer a wide operating range, since the phenomenon of magnetic core saturation which limits the operating range of conventional transformers is not encountered in optical transformers;
it is possible to standardize the apparatus, since it can be used without adaption in all types of shielded installations or shielded stations; and
the apparatus has a wide range of uses.
The state of the art in this field is illustrated by the article "Development of optical instrument transformers" by T. Sawa, K. Kurosawa, T. Kaminsishi, T. Yokota, referenced 89TD380-7-PWRD, IEEE/PES, 1989 Transmission and Distribution Conference, Apr. 2 to 7, 1989.
A first difficulty encountered in installing an optical transformer in a shielded installation is to ensure that there is not too great a difference in potential between certain points on the optical fiber. Such a difference in potential could lead to breakdown.
Another difficulty is to avoid too high a temperature in the optical interfaces associated with the fibers to detect the measurement signal. The temperature inside the cladding of the installations can exceed 90.degree. C. In order to ensure that the apparatus remains intact, and that the measurements are accurate, the temperature inside the associated optical interfaces must not exceed about 70.degree. C.
An object of the invention is to solve those technical problems and to provide an optical transformer whose optical fibers receive a relatively low electrical voltage, and whose associated optical interfaces are in no danger of overheating.
Document FR-A-2,430,112 describes a current transformer constituted by optical fibers embedded in an insulating support. That document does not deal with limiting heat exchange between the fibers and the measuring interface.
Document DE-A-40 25 911 shows optical fibers and a housing inside the metal cladding of a shielded line. Those components are therefore subject to considerable overheating which may prevent them from operating properly.